The field of the present invention relates to data capturing and decoding systems and devices that use imagers.
Solid-state imagers used in data readers, such as hand held and fixed mounted optical code readers (such as barcode readers), may be used to acquire images of moving targets, that is, where the target moves in relation to the imager or the imager moves in relation to the target. Typical solid-state imagers contain an array, typically a two dimensional grid, of electrically coupled light sensitive photodiodes referred to as pixels that convert incident light energy into packets of electric charge. Current solid-state imagers used to capture images of moving targets are generally based on one of the following technologies: (1) charge-coupled device (CCD), (2) complementary metal oxide semiconductor (CMOS) imager with global shutter, or (3) CMOS imager with a mechanical shutter and global reset. The foregoing technologies are commonly used for gathering information from a moving target with a solid-state imager because each technology enables all of the imager's pixels to be exposed to light during the same time period to collect light energy and transform the light energy into an electrical charge. The foregoing technologies also permit all of the pixels to be exposed for a relatively short time that is sufficient to produce a stop-motion image of a moving target, in other words, an image with sharp features.
Because the specialized construction of CCDs is more complicated than the construction of CMOS imagers, and because CMOS imagers may be built in high volume wafer fabrication facilities used for related technologies such as microprocessors and chip sets, CMOS imagers cost less than CCDs. In addition to lower cost, the common fabrication processes used to create CMOS imagers permits a CMOS pixel array to be integrated on a single circuit with other electronic devices such as clock drivers, digital logic, analog/digital converters and other suitable electronics. The compact structures possible for a CMOS imager may also reduce space requirements and lower power consumption.
CMOS based imagers traditionally use rolling shutters to expose pixels in the sensor array. With a rolling shutter, rows of pixels are cleared, exposed, and read out in sequence. During integration, a row of pixels is exposed to light energy and each pixel builds an electric charge corresponding to the amount and wavelengths of light impinging the pixel. Because the rows are activated and read out in sequence, there is an elapsed time between when the first row integrates and when the last row integrates. Because of the elapsed time between when the first row begins to integrate and when the subsequent rows begin to integrate, a CMOS imager with a rolling shutter will likely capture an image of a moving target that is slanted as illustrated in FIG. 1A. The present inventors have realized that to capture images of high speed motion while avoiding motion blur with current CMOS imagers, a short exposure time is needed. Furthermore, the present inventors have realized that a very high intensity light source is required to provide sufficient illumination intensity on the pixels when a short exposure is used. Thus, the present inventors have realized that a CMOS with a short exposure and high intensity light source is not suited for applications near human operators or in battery powered applications.
A current CMOS imager with a rolling shutter used to capture an image of a moving target in relatively low light conditions will likely require relatively long integration times. Relatively long integration times may cause blurred images as illustrated in FIG. 2.
Current data readers using CMOS imagers may require either a global shutter or a mechanical shutter with a global reset to capture a non-slanted, non-blurry, high contrast image of a moving target. Global shutter imagers have a different pixel structure than rolling shutter imagers. In a global shutter imager, all pixels are exposed to light simultaneously during the exposure time. At the completion of the exposure time, the signal captured by each pixel is transferred to a storage area located within the pixel region (a capacitor). During readout, the stored signal is transferred. Since all pixels are exposed at the same time, a “snap” shot of a moving target can be captured. The light source need only be enabled during this short exposure time (much like the flash of a digital camera), making the required light intensity much lower than for a conventional rolling shutter system.
A mechanical shutter has substantially the same effect as a global shutter, but instead of being a part of the solid-state device, a mechanical shutter is a mechanical system that periodically permits light to impinge the rolling shutter CMOS imager. Thus, a rolling shutter CMOS imager with a mechanical shutter may start integrating the first row with the shutter closed, that is, blocking light from impinging the CMOS. The integration time for each row is long enough to permit all of the rows to be integrating during the same time period. After all of the rows are integrating, the mechanical shutter is opened for a short time to expose the pixels to light energy. The mechanical shutter is then closed. A global reset can be used in some rolling shutter CMOS imagers, which starts the exposure of all rows in the imager at the same time. The mechanical shutter is then open and shut and the rows are sequentially read out. Other rows of the image continue to expose until they are read out, but with the mechanical shutter closed, no additional light is integrated on these rows.
The present inventors have recognized that both a global shutter and a mechanical shutter with global reset add complexity and cost to CMOS imagers, thus bringing CMOS imagers closer in cost to CCD imagers. The present inventors have also recognized that mechanical shutters are typically used with a still camera, may only occasionally be operated, and that the complex moving parts tend to wear out and are not suited to high speed imaging, for example, imaging associated with a data reader such as an optical code reader which requires continuous activation of a mechanical shutter for extended periods of time.
The present inventors therefore recognized a need for an improved data reader using a rolling shutter CMOS imager to capture images of moving targets.